The present invention relates to a method of sensing position using magnetoresistors.
It is well known in the art that the resistance modulation of magnetoresistors can be employed in position and speed sensors with respect to moving magnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456). In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object moving relative and in close proximity to the MR, such as a toothed wheel, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth of the moving target wheel is adjacent to the MR than when a slot of the moving target wheel is adjacent to the MR.
Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position formation during cranking. Various combinations of magnetoresistors and single dual track toothed or slotted wheels (also known as encoder wheels and target wheels) have been used to obtain this information (see for example U.S. Pat. Nos. 570,016, 5,731,702, and 5,754,042).
The shortcoming of MR devices is their temperature sensitivity. They have a negative temperature coefficient of resistance and their resistance can drop as much as 50% when heated to 180 degrees Celsius. Generally, this led to the use of MR devices in matched pairs for temperature compensation. Additionally, it is preferable to drive MR devices with current sources since, with the same available power supply, the output signal is nearly doubled in comparison with a constant voltage source.
To compensate for the MR resistance drop at higher temperatures, and thus, the magnitude decrease of the output signal resulting in decreased sensitivity of the MR device, it is also desirable to make the current of the current source automatically increase with the MR temperature increase. This is shown in U.S. Pat. No. 5,404,102 in which an active feedback circuit automatically adjusts the current of the current source in response to temperature variations of the MR device. It is also known that air gap variations between the MR device and ferromagnetic materials or objects will affect the resistance of MR devices with larger air gaps producing less resistance and decreased output signals.
What is needed is a position sensor capable of self compensation over wide ranges of temperature and air gaps, including tilts.
The present invention is a position sensor system capable of self compensation over wide temperature ranges and air gaps, including tilts. It employs three matched MRs (also commonly referred to as MR elements) with either one common bias magnet or separate bias magnets. The MRs are aligned in the direction of movement of a magnetic target. The middle MR is the actual position sensor. The two outer MRs serve as reference sensors which sense the magnetic field at the limits of the position sensing range. The cooperating magnetic target assures that one of the two outer MR elements is always exposed to some maximum magnetic field, BMAX, corresponding to a position XMAX, and the other MR element is always exposed to some minimum magnetic field, BMIN, corresponding to a position XMIN, such that BMINxe2x89xa6BXxe2x89xa6BMAX, corresponding to XMINxe2x89xa6Xxe2x89xa6XMAX, where BX is the magnetic field measured by the middle MR and varies with the position, X, of the target.
The actual position, X, is computed assuming a linear relation between MR resistance in the magnetic field range from BMIN to BMAX and the position, X, of the target. This is accomplished by making the magnetic field a linear function of position in the range XMIN to XMAX and using MRs whose resistance is a linear function of magnetic field in the range BMIN to BMAX.
Alternatively, the shape of the magnetic field profile can be tailored to the MR characteristics in the operating magnetic field range BMIN to BMAX to yield the desired linear relation between MR resistance and position X. That is, the sensor always operates on the linear part of the MR resistance versus magnetic field characteristic curve in the magnetic field range BMIN to BMAX.
Accordingly, it is an object of the present invention to provide a magnetic position sensor system capable of self compensation over wide ranges of temperatures, air gaps and tilts.